This invention relates generally to determination of the power signal frequency for use in protective relay operations, and more specifically concerns a system for maintaining the accuracy of the power system frequency determination if one or more of the individual phase voltage poles are open, or if there are other error conditions in the power system or power system instrumentation transformers.
Protection functions for an electric power system, such as carried out by a protective relay, other protection or monitoring device, often require an accurate knowledge of the power system frequency, i.e. the frequency of the power signal being provided to the system""s customers. Misoperations of a protective relay can occur if the system frequency is not accurately known and tracked, as the frequency may change from nominal frequency. A memory polarized distance element is an example of a protective element in a relay which can misoperate if the system frequency is not accurately tracked. Accurate frequency information is important in a number of protection operations.
Power system frequency can be affected by a number of factors, including operating stress on the system due to high demand, as well as other factors, such as a sudden mismatch of generation to load caused by the liberation of a system fault and the resulting increase in system transfer impedance. Frequency is generally determined by selecting a single phase of the voltage on the power line, usually A-phase, and then making a system frequency determination from that voltage signal, based on the frequency of that single phase voltage. The frequency determination can be accomplished by any one of a number of conventional, well known frequency determining techniques.
Generally, such a system works well. However, if the selected phase voltage is de-energized, such as if the pole opened due to a single pole tripping operation, or if there is a fuse failure of the VT (voltage transformer) supplying that phase voltage to the device or if other factors produce a frequency distortion of that single phase voltage, the ability of the present system to produce an accurate determination of system frequency is severely impaired, if not eliminated. For instance, an open pole or blown VT fuse will result in a lack of frequency determination. The occurrence of these events does not, however, remove the protective device from service nor decrease the need for these devices to fulfill their functions of protection and monitoring.
Hence, it is desirable to increase the reliability and accuracy of power system frequency determinations, because of the importance of such frequency information to the correct operation of many protective functions with protective relays.
Accordingly, the present invention is a system for determining the power system frequency which is used in the protective relay operations for protection and/or monitoring of the power system, comprising: means for obtaining three phase voltages on the power line by elements which are local to the protective relay; a calculating circuit for producing a composite voltage of the three phase voltages in accordance with a preselected equation; and means for estimating the power system frequency from the composite voltage, wherein said power system frequency is used by the protective relay in its protective functions for the power system.